Coldplay fans have been spooked after the coloured wristbands which turn stadiums into a spectacle of lights during the band's shows suddenly began flashing of their own accord, days after the event.
Producers of Xylobands, given free to two million gig-goers on Coldplay's tour, have moved to assure concert-goers, who took wristbands home, that they have no sinister powers.
The Xylobands use ultra-low-power microcontrollers to receive and process wireless signals that trigger the wristband's LEDs to light up in sync with the music and stage lights show.
They cannot be reactivated outside of the show – in theory. But fan Denise wrote on the band's web forum: "My Xyloband woke me up in the middle of the night and it's still glowing."
Another wrote: "Really weird, my dad's white Xyloband just started flashing again." A Belgian concert-goer wrote: "Mine is alive! It started flickering about an hour ago."
A respondent said that the only way to stop the glowing was by jamming a pencil clip into the unit.
Clive Banks, co-founder of RB Concepts which developed the devices, said the "afterglow" is no cause for concern.
"There's no mind control or tracking, they are just for fun," he said.
The devices may be activated by their proximity to a Coldplay concert.
"Our record transmission is over 2km," Mr Banks said. "It depends on buildings but the straight line distance is quite a range. There might be some faulty units going off."
Chris Martin's band has invested in RB Concepts, and the Devon firm has received hundreds of requests to use the devices, from weddings to US sports franchises.